1. Field of the Invention
The invention relates generally to an angular position detector and a rotary electric device drive unit including the same. More specifically, the angular position detector of the invention can be configured at low cost, and can accurately detect the angular position of a rotor in a rotary electric device even when the rotary electric device runs at a high speed.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-2001-86786, Japanese Patent Application Publication No. JP-A-07-79589, Japanese Patent Application Publication No. JP-A-2002-350180, and Japanese Patent Application Publication No. JP-A-2004-12387 describe motor drive units for driving a motor, where the angular position of a rotor is detected to facilitate the continuous rotation of the rotor in the motor within a rotating magnetic field. A resolver attached to the rotating shaft of the motor detects the angular position of the rotor.
FIG. 13 schematically illustrates the block diagram of a conventionally used angular position detector. As shown in FIG. 13, a resolver 302 detects the angular position of a rotor in a motor M, and transmits analog signals SIN and COS indicating the corresponding positions of the rotor to a R/D converter 306. The R/D converter 306 converts the analog signals SIN and COS transmitted from the resolver 302 into digital signals, and transmits the digital signals to a Central Processing Unit (CPU) of a microcomputer. Based on the digital signals received, the CPU produces a drive signal for supplying alternating current, used to produce a rotating magnetic field, to a stator coil provided on the periphery of the rotor. The CPU then transmits the drive signal to an inverter (not shown). The inverter supplies a predetermined alternating current to each phase of the stator coil at a predetermined time according to the drive signal transmitted from the CPU. Thus, the stator coil produces a rotating magnetic field, and the rotor rotates within the rotating magnetic field produced by the stator coil.
In this case, the R/D converter 306 is a dedicated integrated circuit (IC) with complex circuitry including a capacitor-resistor (CR) filter circuit which removes noise components from the signals SIN and COS. However, the inclusion of such circuitry in the R/D converter 306 greatly adds to the cost of the component, and consequently increases the total production cost of the motor drive unit.
However, such excessive costs can be avoided by using angular position detectors that include a microcomputer with a resolver interface integrated with a CPU, which functions as a R/D converter, instead of including a component as costly as an actual R/D converter.
For example, Japanese Patent Application Publication No. JP-A-2002-350180 describes a resolver signal processor which includes a resolver interface that converts signals from a resolver into digital signals; and a CPU that processes the digital signals indicating the rotational angle of the rotors. The resolver interface and the CPU are directly connected to each other through an internal bus.
Japanese Patent Application Publication No. JP-A-2004-12387 describes an angular position detector that is incorporated in an electric motor control device. The angular position detector also produces amplitude ratio signals TAN and COT based on signals SIN and COS transmitted from a resolver, and detects a rotational angle of a rotor based on the amplitude ratio signals TAN and COT.
With the technologies described above, the cost for producing the angular position detectors can be reduced, because R/D converters are not used. In addition, because the resolver interface and the CPU described in Japanese Patent Application Publication No. JP-A-2002-350180 are directly connected to each other, the resolver signal processor excellent in computing speed and noise immunity can be realized.
With the above-mentioned technologies, however, a large amount of sample data is needed to accurately detect the angular position of the rotor when the signals SIN and COS are converted into digital signals to accurately even through the noise present in the signals SIN and COS is removed. As a result, the processing load placed on the microcomputer increases due to the sheer quantity of sample data, which may cause delays in response that causes the detected rotational angle to deviate from the actual rotational angle.
Accordingly, the control means for the motor drive unit requires a microcomputer with high processing power. As the motor speed increases, the processing power required of the microcomputer also increases. Accordingly, when the motor runs at a relatively high speed, it is difficult to perform the above-mentioned computation for detecting the rotational angle due to the lack of sufficient processing power. As a result, use of the angular position detector is limited only to conditions where the motor runs at a relatively low speed, namely, where adequate processing power is available.
Meanwhile, because compact motors becoming increasingly prevalent due to growing demand, it is necessary for the motor drive units run the compact motors at a higher speed in order to achieve the same output. To meet such demands, the angular position detectors are required to detect the angular positions of the rotor even when the motor runs at a high speed.
To fulfill such a requirement, employing a microcomputer with greater processing power as the control means may be effective. However, using a microcomputer with greater processing power also increases the total cost for producing the angular position detectors.